Camera, image sensor, and method for decreasing undesirable dark current
A method for lowering dark current in an image sensor pixel, the method includes the steps of providing a photosensitive area for receiving incident light which is converted into a charge; providing a gate for transferring charge from the photosensitive area; wherein the gate is held at a voltage which will accumulate majority carriers at a semiconductor-dielectric interface during integration for the photosensitive area. Alternatively, a potentail profile can be provided under the gate to drain the dark current away from the photogeneration diffusion.
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The invention relates generally to the field of image sensors and, more particularly, to such image sensors in which undesirable dark current is substantially eliminated.
BACKGROUND OF THE INVENTIONAs is well known in the art, dark current is a significant limitation of the performance of image sensors, particularly CMOS image sensors. A typical image sensor includes a substrate having a photosensitive area or charge collection area for collecting charge, and a transfer gate for transferring charge from the photosensitive area to either a charge-to-voltage conversion mechanism, such as a floating diffusion in a CMOS image sensor, a transfer mechanism in a charge-coupled device image sensor or to a reset mechanism. A dielectric is positioned between the gate and the substrate, and the area of contact between the two areas is generally referred to in the art as the semiconductor/dielectric interface. During certain stages of image capture, such as integration, electrons not associated with the photosensitive process that captures the electronic representation of the image, i.e., the photo-generation process, accumulate in certain portions of the sensor, such as adjacent gates, and inherently migrate into the photosensitive area. These electrons, a portion of what is called dark current, are undesirable as they degrade the quality of the captured image.
It is known that a pinned photodiode includes substantially all the above-described devices except as described hereinbelow. In this regard, pinned photodiodes include a photosensitive area with a pinned layer spanning the photosensitive area. Pinned photodiodes are known to decrease dark current in photosensitive areas. However, dark current still exists from adjacent gates.
Consequently, a need exists for substantially eliminating dark current associated with adjacent gates and other similar structures.
SUMMARY OF THE INVENTIONThe present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a method for lowering dark current in an image sensor pixel, the method comprising the steps of providing a photosensitive area for receiving incident light which is converted into a charge; providing a gate for transferring charge from the photosensitive area; wherein the gate is held at a voltage which will accumulate majority carriers at a semiconductor-dielectric interface during integration for the photosensitive area.
An alternative means to overcoming one or more of the problems set forth above is presented where the potential profile under the adjoining gate is created so that dark current related to the gate is drained away from the photogeneration diffusion.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
ADVANTGEOUS EFFECTS OF THE INVENTIONThe present invention has the advantage of substantially eliminating dark current adjacent gates and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to
Undesirable dark current is generated both in the photosensitive area 20 and along the charge transfer channel 50. Typically, a high rate of dark current generation occurs both at the semiconductor/dielectric interface 42 adjacent to the photosensitive area 20 and at the semiconductor/dielectric interface 40 under the gate 10 due to the high rate of generation resulting from interface states. The dark current from the interfaces 40 and 42 is the dominant source of dark current flowing into the charge sensing node 22. It is noted that the charge sensing node 22 may be replaced by a reset node resulting in the same behavior. For purposes of brevity in the present invention, the implementation with the charge sensing node 22 will be discussed.
Referring to
In this configuration, a dominant source of dark current is at the semiconductor/dielectric interface or surface 40 under the gate 10. The present invention presents a means of surpressing this dark current by biasing the gate 10 to a potential so that the semiconductor at the interface 40 becomes accumulated with free carriers of the majority doping type. The dark current generation occurs because the defects are in an non-equilibrium state, and this accumulation supresses this generation by returning the region where the highest quantity of defects occur to local equilibrium.
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In the present invention, an additional mechanism, in addition to the above described biasing, is disclosed to eliminate the contribution of dark current from the interface under the gate 10 and the charge transfer channel under the gate 56. Referring to
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The invention has been described with reference to preferred embodiments. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.
PARTS LIST
- 10 gate
- 15 dielectric
- 20 photosensitive or charge collection area
- 22 charge-to-voltage conversion node (or referred to alternatively as diffusion or charge sensing node)
- 30 substrate/semiconductor
- 32 heavily doped diffusion
- 40 semiconductor/dielectric interface
- 42 semiconductor/dielectric interface
- 50 path or gate controlled charge transfer channel
- 52 potential extremium or collection potential
- 54 charge-to-transfer potential transition or collection-to-transfer potential transition
- 56 gate-associated charge transfer channel or gate channel potential
- 58 destination potential
- 70 pixels
- 75 image sensor
- 80 on-chip circuitry or generation source
- 90 off-chip or external circuitry
- 200 camera
Claims
1. A method for lowering dark current in an image sensor pixel, the method comprising the steps of:
- (a) providing a photosensitive area for receiving incident light which is converted into a charge; and
- (b) providing a gate for transferring charge from the photosensitive area; wherein the gate is held at a voltage which will accumulate majority carriers at a semiconductor-dielectric interface associated with the gate during integration for the photosensitive area.
2. The method as a claim 1 for comprising the step of providing a charge-to-voltage conversion node that receives the charge from the photosensitive area.
3. The method as a claim 1, wherein step (b) comprises providing an on-chip generation source for generating the voltage.
4. The method as a claim 1, wherein step (b) comprises providing an external voltage source for generating the voltage.
5. The method as a claim 1 further comprising the step of providing a gate channel potential so that charge generated at a surface adjacent the gate is directed away from the photosensitive area.
6. The method as a claim 1 for comprising the step of providing a negative voltage as the voltage that will accumulate carriers at a semiconductor-dielectric interface.
7. The step as a claim 1 further comprising the step of providing silicon as the semiconductor and silicon dioxide as the dielectric.
8. A method for lowering dark current in an image sensor pixel, the method comprising the steps of:
- (a) providing a substrate;
- (b) providing a photosensitive area in the substrate for receiving incident light that is converted into a charge; and
- (c) providing a gate for transferring charge from the photosensitive area; wherein step (a) includes providing the substrate substantially adjacent the gate with an impurity concentration that transfers dark current away from the photosensitive area and into a diffusion on an opposite side from the photosensitive area during integration for the photosensitive area.
9. An image sensor pixel that substantially eliminates dark current, the sensor comprising:
- (a) a photosensitive area for receiving incident light that is converted into a charge; and
- (b) a gate for transferring charge from the photosensitive area; wherein the gate, when at a predetermined voltage, will accumulate majority carriers substantially at a semiconductor-dielectric interface associated with the gate during integration for the photosensitive area.
10. The image sensor as in claim 9 further comprising a charge-to-voltage conversion node that receives the charge from the photosensitive area.
11. The image sensor as in claim 9, further comprising an on-chip generation source for generating the voltage as the predetermined voltage.
12. The image sensor as in claim 9 further comprising an external voltage source for generating the voltage.
13. The image sensor as in claim 9, wherein the predetermined voltage is a negative voltage that will accumulate carriers at a semiconductor-dielectric interface.
14. The image sensor as in claim 9, wherein silicon is provided as the semiconductor and silicon dioxide is provided as the dielectric.
15. An image sensor pixel comprising:
- (a) a substrate;
- (b) a photosensitive area in the substrate for receiving incident light that is converted into a charge; and
- (c) a gate for transferring charge from the photosensitive area; wherein the substrate substantially adjacent the gate includes an impurity concentration that transfers dark current away from the photosensitive area and into a diffusion on an opposite side from the photosensitive area during integration for the photosensitive area.
16. A camera comprising:
- an image sensor pixel that substantially eliminates dark current, the sensor comprising: (a) a photosensitive area for receiving incident light that is converted into a charge; and (b) a gate for transferring charge from the photosensitive area; wherein the gate, when at a predetermined voltage, will accumulate majority carriers substantially at a semiconductor-dielectric interface during integration for the photosensitive area.
17. The camera as in claim 16 further comprising a charge-to-voltage conversion node that receives the charge from the photosensitive area.
18. The camera as in claim 16, further comprising an on-chip generation source for generating the voltage as the predetermined voltage.
19. The camera as in claim 16 further comprising an external voltage source for generating the voltage.
20. The camera as in claim 16, wherein the predetermined voltage is a negative voltage that will accumulate carriers at a semiconductor-dielectric interface.
21. The camera as in claim 16, wherein silicon is provided as the semiconductor and silicon dioxide is provided as the dielectric.
22. An image sensor pixel comprising:
- (a) a substrate;
- (b) a photosensitive area in the substrate for receiving incident light that is converted into a charge; and
- (c) a gate for transferring charge from the photosensitive area; wherein the substrate substantially adjacent the gate includes an impurity concentration that transfers dark current away from the photosensitive area and into a diffusion on an opposite side from the photosensitive area during integration for the photosensitive area.
Type: Application
Filed: Oct 28, 2004
Publication Date: May 4, 2006
Patent Grant number: 7973836
Applicant:
Inventors: R. McGrath (Pittsford, NY), Edward Nelson (Pittsford, NY), Robert Guidash (Rochester, NY), Charles Stancampiano (Rochester, NY), James Lavine (Rochester, NY)
Application Number: 10/975,865
International Classification: H04N 5/217 (20060101); G03G 15/056 (20060101); G03G 15/24 (20060101);